Electrostatic fluidized bed coating is, of course, a conventional and widely used technique for depositing particulate materials upon a great diversity of workpieces. Typical of the apparatus used for that purpose are the devices disclosed in Knudsen U.S. Pat. No. 3,916,826 and in Karr U.S. Pat. No. 4,030,446. While electrostatic coating with such devices is highly efficient, effective and safe, fluidized bed coating in general is not without its limitations and difficulties.
A particularly difficult problem relates to the attainment of uniform deposits upon all surfaces of the articles being coated. Such problems are due in part to the effects of the workpiece configuration upon the nature of the coating produced. Thus, a non-uniform workpiece will tend to develop an irregular deposit, particularly when, for example, the article has sharp edges or elements of relatively small dimension. But even when the workpiece is of entirely uniform configuration and is free of edges (e.g., when it is a length of round wire), the proximity of other workpieces will usually have an effect. Thus, when a plurality of wires are coated simultaneously, stopping or removing one of them will usually significantly change the characteristics of the deposit produced upon the others. This is highly undesirable in an automated operation, such as is, as a practical matter, necessary in the commercial production of insulated wire. But even when only one wire is involved, still a problem exists in producing the high degree of uniformity, from point to point along the full length of the wire, required for certain applications.
In those instances in which coatings are produced upon articles moving horizontally (or substantially so) above or through a fluidized bed, the difficulty of producing uniformity is compounded by the fact that rarefaction occurs upwardly within the cloud. As a result, the upper surfaces of the articles are exposed to less powder than are the lower portions, and therefore the deposits developed thereon tend to be thinner. The generally upward movement of the particles of the cloud, under the influence of the gas passing upwardly through the porous support plate of the fluidized bed, also favors the development of heavier coatings on lower surfaces.
Attempts have been made to compensate for the foregoing characteristics of electrostatic fluidized bed coating, such as through the use of appropriate masking devices to block those surfaces which would otherwise tend to receive disproportionately large amounts of the powder. Similarly, in Westervelt et al U.S. Pat. No. 4,011,832, build control means is disclosed for the purpose of selectively achieving either a uniform or a nonuniform deposit upon a workpiece, the invention being directed in particular to wire coating. While that approach has considerable merit, it nevertheless involves complexity of design, and tailoring of the equipment to a specific application.
Another difficulty which is inherent in powder coating entails the need for control, so as to minimize the waste of powder and to avoid creating an unhealthy and hazardous work environment. In some instances, these objectives are accomplished by the collection and recirculation of the coating powder, such as with a system of the sort disclosed in Carlson et al U.S. Pat. No. 4,123,175. While such systems are widely used, and are very effective, their installation does entail a considerable capital investment, and requires a significant amount of floor space. Problems of powder loss and contamination have also been dealt with through the use of electrodes, which are appropriately positioned with respect to the cloud and either grounded or charged to attract or repel fugitive particles. Such an approach is taught by Nakaya in U.S. Pat. No. 3,059,613, and by Point in U.S. Pat. No. 3,336,903. While such techniques may have merit, they do not provide the degrees of control and regulation necessary for convenient and effective operation as a practical matter, and especially not for purposes of coating wire on a commercial basis.
Many of the foregoing difficulties encountered in prior art apparatus and methods have been alleviated or avoided by the invention of Donald J. Gillette, set forth in U.S. Application for Letters Patent Ser. No. 114,656, filed on Jan. 23, 1980 and now issued as U.S. Pat. No. 4,297,386, which application is entitled CONTROL GRID IN ELECTROSTATIC FLUIDIZED BED COATER, and is of common assignment herewith. However, further improvement, especially in regard to uniformity of the coating along the length of a continuous length workpiece, are of course desirable.
Thus, it is a principal object of the present invention to provide a novel apparatus, system and method for electrostatic powder coating, by which workpieces, and especially wires of continuous length, can be coated with a high degree of uniformity in the thickness of the deposit from point-to-point along the length of the workpiece.
It is also an object of the invention to provide such an apparatus, system and method in which coating can be effected at reduced voltages, without loss of production speed.
Another object is to provide the foregoing by means that are relatively simple, inexpensive and convenient.